This invention relates generally to homogeneous charge compression ignition engines, and more particularly to homogeneous charge compression ignition engines having cylinders with a high compression space and a low compression space.
Due in part to more stringent government emissions standards, engineers have long been searching for methods to reduce emissions created by diesel engines. One solution that has shown reasonable success in lowering NOx and soot emissions is the use of a Homogenous Charge Compression Ignition (HCCI) engine operating mode. In HCCI operation, fuel is injected into an engine cylinder when the power piston is at or near the bottom dead center position of its compression stroke. As the power piston advances toward its top dead center position, the fuel then mixes with air in the cylinder to form a relatively lean fuel/air mixture, which creates fewer emissions upon ignition.
While HCCI operation has shown some success at reducing diesel engine emissions, there is still room for improving engine utilizing this mode of operation.
For instance, while fuel is injected into the cylinder when the power piston is near the bottom dead center position, ignition of the fuel/air mixture should not occur until the power piston is at or near its top dead center position. When the start of ignition occurs too soon, the fuel and air mixture will burn during the compression stroke of the cylinder piston, resulting in possible engine damage and poor cycle efficiency. In addition, this could lead to high temperatures during the next engine cycle, which could result in another early start of ignition. Thus, control of ignition timing in response to engine operating conditions, such as cylinder temperature and fuel pressure, is critical.
One attempt to control ignition timing is illustrated in U.S. Pat. No. 5,832,880, which issued to Dickey on Nov. 10, 1998, and is entitled Apparatus And Method For Controlling Homogenous Charge Compression Ignition Combustion In Diesel Engines. In Dickey, a closed loop control strategy is disclosed which includes regulation of the timing by controlling an amount of water that is injected into a cylinder of a homogenous charge compression ignition engine to control the start of ignition. While this purported solution may have some viability, there is still room for improvement. For instance, for those engines that do not utilize water-spraying fuel injectors, a solution to controlling timing of ignition events is needed.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, a homogeneous charge compression ignition engine includes an engine housing defining at least one cylinder. A power piston having a raised portion is positioned to reciprocate in the cylinder between a bottom dead center position and a top dead center position. A fuel injector for the cylinder is operable to inject fuel into the cylinder when the power piston is closer to the bottom dead center position than to the top dead center position. The power piston and the engine housing define at least part of a high compression space and at least part of a low compression space when the power piston is in the top dead center position.
In another aspect of the present invention, a method of igniting a homogeneous charge in an engine includes the step of injecting fuel into a cylinder when a power piston is closer to a bottom dead center position than a top dead center position. A homogeneous charge is divided between a high compression space and a low compression space with a power piston. A portion of the homogeneous charge in the high compression space is ignited, at least in part by compressing the portion to a compression ignition point. A remaining portion of the homogeneous charge is ignited, at least in part by fluidly connecting the high compression space to the low compression space.
In yet another aspect of the present invention, a homogeneous charge compression ignition engine includes an engine housing defining at least one cylinder having a centerline. A power piston having a raised portion is positioned to reciprocate in the cylinder between a bottom dead center position and a top dead center position. A fuel injector for the cylinder is operable to inject fuel into the cylinder when the power piston is closer to the bottom dead center position than to the top dead center position. The power piston and the engine housing define at least part of a high compression space and at least part of a low compression space when the power piston is in the top dead center position, wherein the centerline intersects the high compression space.